Various organic electronic devices, including light emitting devices, have been known in the art over the years. For example, U.S. Pat. No. 5,682,043 discloses an embodiment of an organic light emitting electrochemical cell (OLEC)-type device, OLECs typically require low operating voltage and may be suitable for stable and potentially low cost organic light emitting devices. The stability may be related to the fact that the electrochemical doping used in OLECs facilitates charge injection barrier narrowing and reduces the need for low work function metals for charge injection into active layer organic semiconductors. Low work function metals are inherently unstable to corrosion from water or oxygen. This inherent instability can lead to shortened operational lifetimes due to electrode degradation caused by ingress of water from the environment into device packages and potential reaction with the electrode metal. OLECs can achieve low voltage charge injection by using more stable, higher work function metals such as Ag, Al, Ni, or Au, which are more immune to oxidation and/or by using conducting oxides that do not interfere as significantly with charge transport. In their normal form, the active layers of OLECs, such as those described in U.S. Pat. No. 5,682,043, are deposited from a single solution containing light emitting and charge transporting conjugated polymers, for example polyphenylene vinylene polymers, polythiophene-based polymers or polyfluorene-based polymers, in combination with an electrolyte system which typically includes mobile ionic dopants, for example Li triflate (trifluoro methanesulfonate) along with electrolyte forming materials such as polyethylene oxide or similar. Low voltage charge injection in these devices, and relatively high quantum efficiency under bias, is generally attributed to the electric field driven redistribution of electrolyte ions at the cathode and anode interfaces of the OLEC with its adjacent electrode. The ionic dopant redistribution serves to electrochemically dope the conjugated semiconductor active layer materials near the interfaces, leading to narrowing of the injection barriers between the electrode work functions and the molecular energy levels in the active layer semiconductors. This effectively lowers the required bias to inject electrons and holes into the active layer leading to lower voltage electroluminescence in the device when significant and relatively balanced electron and hole currents recombine at luminescent sites in the active layer.
Conventional organic light emitting diodes (e.g., the device shown in U.S. Pat. No. 4,539,507) rely on near-ohmic matching of the work function of the electrodes and the energy levels within the active layers of the device. These devices are often composed of multiple layers of organic semiconductor-based materials with different energy levels. In contrast to that, OLECs have the advantage that charge injection at low bias can be achieved from metals that are less well-matched in terms of work function than what conventional OLEDs require. Of particular interest is the elimination of the need for low work function cathode materials in OLECs at the electron-injecting interface. Low work-function cathode metals, such as Ca, Li, Cs, Ba, and Mg, which are commonly used in conventional devices, including polymer OLEDs (e.g., devices described in U.S. Pat. No. 5,247,190), are typically unstable to oxidation or other degradation mechanisms, particularly in the presence of moisture or oxygen. This warrants restrictions on the processing environments required for devices containing these materials. Similarly, use of low work-function metals necessitates higher encapsulation requirements and/or shorter storage lifetimes because impurities like water or oxygen leaking into the device can lead to spontaneous cathode degradation. For reference, an interested reader may review U.S. Pat. No. 6,522,067.
To date, most OLEC devices have been prepared using conventional deposition approaches, such as spin coating of active layers and vacuum evaporation of thin film cathodes, which are similar to the processes used in preparing conventional reactive cathode OLEDs. These methods include evaporation of metals, such as Al and Ag, to form continuous, specular and relatively impermeable cathodes adjacent to the active layers of the devices.
Recently, increasing attention has been given to printing and coating based approaches for the fabrication of semiconductor thin film devices in general, and, in particular, for the fabrication of organic semiconductor thin films devices. Printing and coating-based manufacturing approaches allow lower cost, higher volume production of device over larger areas and on flexible substrates. Controlling doping parameters is particularly pivotal in the manufacturing of light emitting organic devices as it allows for the use of printable, air-stable, and air-processible cathode materials. Examples of such a device can be found in U.S. Pat. No. 6,605,483.
To realize a practically useful device, it is necessary to prevent impurity (e.g., water and oxygen) ingress into the device during the storage and product lifetime, as oxidation of device materials can severely affect device performance and limit device lifetime. This applies even in the case of OLECs that do not use low work function, reactive cathodes, as light emitting polymers, small molecules, conjugate charge transport materials and other active materials are still prone to oxidative degradation, particularly in the excited state, i.e. when the device is energized and polaronic or excitonic species are present in the active materials. These excited states lower the barrier to reaction, and/or increase the oxidation rate. High excited state densities can also occur when devices are exposed to light or heat, such as in case of photovoltaic cells, photodiodes or other similar sensors. Furthermore, high concentrations of moisture, oxygen or other detrimental species may exist inside a device as manufactured, possibly due to residual impurities in starting materials or impurities absorbed into the device during its manufacture. In conventional OLED devices, devices are typically fabricated on glass substrates, which are rigid and have low water and oxygen permeability. As mentioned previously, the active layer of the device is locally covered by a vacuum evaporated contact, thereby sandwiching the active layer materials between low permeability glass and metal. Devices are typically edge-sealed with epoxies to a backside (assuming bottom emission) encapsulation metal can or glass sheet forming a fully-encapsulated package. In order to control the moisture levels within the package during storage or product lifetime, a relatively small (compared to the total device or package area) getter or desiccant patch may be placed on the encapsulation can or sheet. In some cases, desiccant may be dispensed inside the edge seal perimeter area.
Considering the low aspect ratio of these OLED device structures (assuming the active device as separate from its packaging), there is usually a very restricted route for water, moisture and/or solvents in or out of the completed device active layers. For example, the routes may be via relatively small area edges of the device. Devices are typically tens of microns or more in lateral extent and not thick either. For certain lighting or photovoltaic devices, the lateral dimensions expand to centimeters while the thickness remains in the 100 nm range. These general dimensions mean that moisture from starting materials or processing that is trapped in this conventional OLED layer between the low permeability glass and metal (e.g. as disclosed in co-pending co-owned International patent application no. WO2010/141519, titled, “Encapsulation Process and Structure for Electronic Devices”) cannot easily diffuse to the getter and be absorbed. This prevents establishment of a gradient of moisture (or other impurity) between the device and getter, thus preventing lowering moisture concentration of the active device layers by the getter. This is particularly important for printed devices (e.g., as disclosed in co-pending co-owned International patent application no. WO2010/141493, titled, “Formulations for Improved Electrodes for Electronic Devices”), where low solids, load inks, solvents and layers may contain significant amounts of residual water. Solid loading of typical organic semiconductor inks may be a significant factor, as in extreme cases, it is possible to have as high as 100% amplification factor in impurity concentration upon drying.
Getters may also include solvent getters (activated carbon, zeolite, etc.) which can remove residual solvent and allow for lower drying temperatures and drying times and a latent drying effect. A latent effect means removal of solvents after the device was fabricated. This can provide manufacturing advantages as solvent drying is often a rate limiting step in manufacturing depending on the time and oven size required for long drying periods in high throughput process lines. Inclusion of solvent getters can lower extended solvent annealing times that is otherwise needed, and can reduce overall manufacturing time, reduce process line footprint and lower costs (due to reduced oven size) and drying bottlenecks.
Flexible OLEDs, PVs, TFT arrays, and sensors add a new encapsulation challenge. Plastic substrates are attractive options for substrate and/or encapsulation films. To reach reasonable barrier properties for most organic electronics devices, 10−2 g/m2/day or less water vapor leakage rates are required to prevent damaging moisture levels due to ingress within typical product lifetimes. This number may vary depending on particular device configurations. For example, typical leakage rates are 10−3 g/m2/day to 10−5 g/m2/day for certain OLEDs manufactured by the company formerly known as Add-Vision, which was based in Scotts Valley, Calif.; and, 10−6 g/m2/day for reactive cathode OLEDs for devices made by Philips, Vitex etc. Bare plastics are not able to achieve these levels and barrier films are required to achieve acceptable transparent barrier substrates or encapsulation films. Typically, these substrates include layer or multilayer sets of inorganic oxide or nitride films. Theoretically, barrier properties of even very thin oxide films can be very high. As a practical matter, the main leakage path in thin transparent barrier films is through small localized defects and pinholes in the film. This is particularly damaging for conventional reactive cathode OLEDs as these local leakage points can cause local cathode corrosion and EL emission dark spots even if they overall average area leakage rate is below a specified acceptable level. This could also be an issue for some moisture or oxygen reactive injection layer or emitters, such as some organo-metallic phosphors, which may spontaneously react with moisture or oxygen.
In printed, stable cathode devices such as OLEDs manufactured by former Add-Vision (in Scotts Valley, Calif.) utilizing non-reactive emitters, spontaneous oxidation of cathodes in storage is usually not a major issue. However, the use of a non-permeable cathode, such as a typical continuous evaporated metal, would prevent diffusion of moisture and oxygen to the getter and may result in local buildup of high levels of moisture or oxygen in the active layer of the device.
Further, a preferred method for a fully flexible encapsulated device is to produce a voidless conformal structure (e.g. devices described by International Application No. WO2010/141519) for mechanical stability and dimensional stability under flexing. In this case, mobility of species within the package is reduced by the fact that all materials between the front and back encapsulation surfaces are filled with adhesive or other material (i.e. substantially no voids). Diffusion through these materials is slow. In this case it is advantageous to place the getter immediately behind the OLED (either by printing, placing or laminating the getter immediately behind the OLED or on the encapsulation surface immediately opposite the OLED). Further, to maintain OLED EL, voltage rise and aging uniformity, it is advantageous to have a one-to-one correspondence between OLED active area and getter area. It is therefore advantageous to have a thin, patterned getter such as a screen printable getter formulations, such as SAES Getter Drypaste getter inks, which can be arbitrarily patterned to thicknesses ca. 10 microns-100 microns or thicker with repeated printing.
Even in the case where water may ingress through the barrier immediately adjacent to the device and interact with the device, a permeable cathode allows for diffusive access to the getter/desiccant. If the free energy of the impurity in the getter (or equilibrium partial pressure over the getter) is lower than the free energy of the situation where the impurity is in the OLED, in some cases the impurity/OLED interaction can be reversibly overcome and the impurity effectively gettered. This is facilitated with a permeable cathode that allows rapid diffusion due to improved access by the impurities and diffusion towards the getter. Impermeable getters can represent a diffusion barrier. It is also conceived that the electric fields contribute to charged impurity motion and so drift effects may also be enhanced with permeable cathodes.